1. Field of the Invention
The present invention relates to an autofocusing technique for an imaging device that converts an optical image, which is captured via an optical system having a focusing function, into a video signal by means of an imaging system.
2. Description of the Related Art
The advance of the digital techniques has boosted the spread of digital video cameras and other electronic cameras. The electronic camera generally has an autofocusing mechanism that automatically adjusts the position of a focus lens to be focused on a subject. One typical technique adopted for the autofocusing mechanism is hill-climbing control. The hill-climbing control obtains an AF evaluation value, while moving back and forth the position of the focus lens along an optical axis. The AF evaluation value represents a high frequency component of a brightness signal that is generated by a CCD or another imaging element and is detected by a wave detector. The position of the focus lens giving a peak of the AF evaluation value is specified as a focus position, and the focus lens is controlled to be set in this specified focus position. An edge area of a captured image generally has a large fraction of high frequency component. Among captured images in a fixed depth of field, the image with the greater AF evaluation value is expected to have the sharper edge or the higher degree of focus. This is the reason why the high frequency component is used for evaluation of focusing. Known examples of the electronic camera with such control mechanism are disclosed in JP-A-1-125065, JP-A-7-87377, and JP-A-2006-79069.
The AF evaluation value obtained in the hill-climbing control, however, includes high frequency noise, which may interfere with accurate autofocusing control. There would thus be a demand for a technique of efficiently and accurately performing autofocusing control in view of potential influence of noise.